Many of the home electronic products, particularly electronics such as small appliances, cordless phones and laptop computers use external power supplies called “wall-packs” or “bricks.” These power supplies convert 120 volts of alternating current (AC) to the low voltage direct current (DC) used by most home electronic products. The DC voltage used by most target devices is commonly between 3V-14V. Most external power supplies are a linear power supply that use a transformer comprising two coils of wire and a magnetic field to lower voltage to the desired level. A bridge rectifier then converts the low voltage AC to DC. Other circuitry smoothes and filters the current to product a flat DC waveform. In operation, the transformer coils produce heat contributing to lost energy and a reduction in the effective life of the linear power supply.
Linear power supplies continue to draw power after the attached device using the power supply has been turned off. The amount of energy used by a power supply when the attached device is turned off is called the “standby power load.” The standby power load generates waste heat which adds to the cooling load of a structure, and shortens the life of the power supply.
Chris Caldwell and Travis Reeder, in a May 2002 National Resources Defense Council publication, “Power Supplies: A Hidden Opportunity for Energy Savings,” described the standby power load problem and known solutions to the problem. One known solution for dealing with the problem of standby power load eliminates the standby power load by manually turning off the power supply. Power supplies can be used on a power strip with a switch, or the power supply can have an integrated power switch so that consumers of electronic devices can turn off or unplug the power supply when it is not in use. While this solution may work for infrequently used devices such as battery chargers, it is not practical for other devices, such as a fax machine or an answering machine, that need to be ready to operate at any time.
Another known solution for addressing standby power load involves “switching” power supplies that use internal solid state electronics to switch between high (peak) load and low (standby) load modes depending on the needs of the attached device. Switching between high and low load modes can be achieved either by using multiple transformers corresponding to the different power demands, or by using “pulse width modulation.” Pulse width modulation delivers power in a series of brief pulses. The switching power supply creates only the number of pulses necessary to meet the power demand, and skips pulses when demand is low. These switching power supplies reduce the standby power load, but do not totally eliminate the standby power load. Power supplies with pulse width modulation can increase peak load efficiency by supplying only the amount of power needed at any given time.
While switching between peak and standby modes can reduce the standby power load, a need exists for an intelligent power supply that automatically eliminates standby power load so that no AC power is drawn when the attached device is turned off.